refactor(packet): replace zlib with zstd compression and restructure header

Increase header length to 12 bytes, add CompressType and PacketHeader struct, and implement decode_packet_header. Update CMake to find zstd and link against it, adding Findzstd module.

cmake/modules/Findzstd.cmake

@@ -0,0 +1,22 @@
+find_path(ZSTD_INCLUDE_DIRS
+  NAMES zstd.h
+  HINTS ${zstd_ROOT_DIR}/include)
+
+find_library(ZSTD_LIBRARIES
+  NAMES zstd
+  HINTS ${zstd_ROOT_DIR}/lib)
+
+include(FindPackageHandleStandardArgs)
+find_package_handle_standard_args(zstd DEFAULT_MSG ZSTD_LIBRARIES ZSTD_INCLUDE_DIRS)
+
+mark_as_advanced(
+  ZSTD_LIBRARIES
+  ZSTD_INCLUDE_DIRS)
+
+if(ZSTD_FOUND AND NOT (TARGET zstd::zstd))
+  add_library (zstd::zstd UNKNOWN IMPORTED)
+  set_target_properties(zstd::zstd
+    PROPERTIES
+      IMPORTED_LOCATION ${ZSTD_LIBRARIES}
+      INTERFACE_INCLUDE_DIRECTORIES ${ZSTD_INCLUDE_DIRS})
+endif()